The structure of the commonly used flat type vibrator is illustrated in FIGS. 1a-1b. The eccentric oscillator (1) and coils a and b are glued to the circuit board (2). Then it is injection molded with the oil bearing (4) at the center into a rotor (3). The shaft (5) is held by the lower shell (9). The rotor is then installed on the shaft through the bearing hole. A magnet (8) is adhered onto the lower shell (9) to provide magnetic flux. The electric brushes (10) are welded to the soft circuit board (11). The upper shell (7) covers the rotor and the lower shell. The washers 6a and 6b are installed on the top and the bottom of the rotor bearing, respectively.
FIGS. 2a-2b illustrate a flat type vibrator design disclosed in patent CN1206240A. In order to increase vibration, the coils a, b and c are placed eccentrically. A piece of high density composite engineering plastic (1) is placed at the exterior of the eccentric rotor and extends below the bottom surface of the rotor to increase the eccentric mass. This extended portion extends into the circular space between the magnets (8) and the upper shell (7). The center portion is the resin that has a density less than 1.8 g/m3.
FIGS. 3a-3b illustrate a design that increases vibration disclosed by Korean patent 20-0333424. The eccentric oscillator (1) is placed on the exterior of the rotor rim. It is affixed onto the hard circuit board (2), and is then combined with the center bearing (4) by injection molding.
The problem with the first design is that in order to prevent the oscillator from detaching, the oscillator is injection molded inside the resin, thus limiting the volume of the oscillator and increased the eccentric distance.
In the second design, the increase in the eccentric mass is limited by the composite engineering plastic. The size of the magnets must be reduced because of the extended portion of the rotor, which greatly decreases the magnetic flux of the motor. As a result, the motor performance is greatly reduced.
Although the third design increased the eccentric distance to a certain degree, the increase in the oscillator volume is still limited.
A common problem which exists in all the above designs is that the amount of vibration decreases significantly when the motor size is reduced. Another problem is that the coils and the eccentric oscillator must be adhered with glue before the rotor is molded. If the adhesion is poor, the coils and oscillator may shift in position during molding.
Thus, it is desirable to have an improved vibrator design that increases the amount of vibration within a given size.